In a conventional photolithography process, when dense lines and an isolated line are formed on a wafer, the depth of focus (DOF) margin of the dense lines provides a resolution 1.5 times higher than that of the isolated line. Generally, DOF is determined by checking the focus variation within ±10% from a target when the focus of an exposure apparatus varies. Particularly, it is very important to secure the DOF margin of an isolated line in forming gates of a logic device which requires many isolated lines.
Korean Patent Publication Number 2001-0047253, Hynix Semiconductor Inc., describes a photolithography using a double exposure to accurately pattern the hole-pattern of high density. In the described method, a mask is arranged to open the regions corresponding to the (1, 3, 5, . . . , n-1)th or the (2, 4, 6, . . . , n)th hole-patterns in the hole-regions in a direction in which the distance between the hole-patterns is shorter than that of the other direction. Here, the mask is fixed on the first position to transfer a wafer. The mask is shifted by the distance between the hole-patterns in a direction in which the distance between the hole-patterns is shorter than that of the other direction. Therefore, the mask is fixed on the second position to transfer the wafer.
However, such a conventional photolithography process has a problem that an isolated line pattern cannot secure a DOF margin although it is equal to dense lines in size.